Pigment and paint and method of making same



Patented Feb. 25, 1941 UNITED STATES,

2,233,042 PIGMIENT AND PAINT AND METHOD OF. MAKING SAME Louis E. Barton,Windsor, Conn.

N Drawing.

Application January 23, 1939, Serial No. 252,440

19 Claims. (01. 13467) 7 This invention relates to pigments, theirpreparation and application and more especially to improved opaque whitepigments-the silicates of lead-the method of making and stabilizing themand their application in paint.

In principle, this invention is based upon the process of making leadsilicate pigments by heating a mixture of lead oxide, or a lead compoundwhich upon heating yields lead oxide, and silica under controlledtemperature conditions whereby the reaction to form lead silicate takesplace in the solid phase without fusion, thus ensuring a soft finetexture and otherdesirable physical qualities in the pigment product. Bysuch proc- 15 ess any desired lead silicate such as acid lead silicate(PbSi2O5), normal lead silicate (PbSiOz) and basic lead silicate(PbzSiOi) can be prepared and also lead silicates having otherproportions which apparently are either mixtures of those definitechemical compounds or in case an excess of either lead oxide or silicahas been used 1 the product may contain some free oxide or the excessoxide may exist as a solid solution in the silicate. I have found thatlead silicate pigments so prepared are not stable to the action oflight, acquiring a gray or gray-brown tone or color in a few hours underthe action of direct sunlight or after a longer period in diffusedlight. All of the lead silicate pigments whether acid, normal or basic,are to some extent unstable to light but I have found that theinstability increases with the basicity or content of lead oxide in.thepigment. While such lead silicate pigments make paints which areexceptionally durable when exposed to light and weathenthe unstablecharacteristic of the pigment still persists after I grinding withdrying oil vehicles such as linseed oil, tung oil, and oil varnishes.The resulting white paints upon exposure to light soon acquire a grayishcolor which gradually becomes darker. Such unstable lead silicatepigments are not available for making white or light-tinted paints.

In my co-pending U. S. patent application Serial No. 244,768 filedDecember 9, 1938, I have disclosed a process of stabilizing leadsilicate pigment such as referred to in the preceding paragraph, whichconsists in adding, to a mixture of :a lead compound and silica, a smallamount of a white oxide of one of the elements of group II of 50 theperiodic system and then heating the mixture until the ingredientschemically combine in the solid phase without fusion ofthecharge.

I have now discoveredthat light-stable lead silicate pigments can bemade, which'will not darken and discolor when exposed to light, byadding to the mixture of lead compound and silica a small amount of ahalogen compound of a one of the elements of group II of the periodicsystem the oxide of which is white, and then heating the mixture untilthe ingredients chemically combine ingthe solid phase to form leadsilicate Without fusion. Ofthe class ofi'stabilizers referred to I havefound the halogen compounds of magnesium, calcium, barium, strontium andzinc particularly satisfactory since they are industrially andeconomically available.

I have also discovered that small-amounts of the halogen compounds ofthe white-oxide-forming elements of group II of the periodic system notonly stabilize the lead silicate pigment against discoloration under theaction of light, butalso greatly accelerate the reaction between thelead compound and silica when the mixture is heated to form leadsilicate in the solid phase 7 and furthermore, the reaction can becompleted at lower temperatures than is possible and practicable withoutsuch accelerating component.

The use of the stated halogen compounds, having the dual function ofaccelerating the reaction and stabilizing the reaction product, permitsconsiderable variation in the amount of halogen compound used, thecalcination temperature, the time required to complete the reaction andthe characteristics of the resulting pigment. Thus it is possible tocomplete the reaction between the lead compound and silica in theminimum time with relatively more of the halogen compound or with asimilar amount .to complete the reaction at a lower temperature in asomewhat longer period of time. On the other hand, the pigment may bestabilized with very small amounts of halogen compounds, the time forcompletion of the reaction depending on the temperature, and thusyielding a stabilized lead silicate pigment having a minimum content ofthe stabilizing compound. The lower calcination temperatures,permissible when using a halogen compound as accelerator and stabilizer,

proved hiding power. 7

The mechanism of the reactions whereby small amounts of the halogencompounds referred to, accelerate or catalyze the reaction in theformayield lead silicates of softer texture and imtion of lead silicatesand stabilize the resulting products: is not definitely known, howeverthe observed reactions which take place during operation of the processshould be considered in deciding upon the kind and amount of halogencompound, the calcination temperature and the treatment ofthe calcinedproduct. Partial decompositionof some of the halogen compounds occursduringheating with the lead compound andsilica, with volatilization ofsome of thehaler part of both basic element and halogen re- 'mains inthe calcined lead silicate. The halogen combination, and a part remainswater-soluble.

The amount of water-soluble compound increas- ,ogenfo r example, whenbromides and iodides I are used-and some halogen compounds volatilizepartially-as such under the heating conditions, for example, zincchloride; but, regardless of the amount of halogen compound, the'greateswith the amount of halogen compound used, hence if larger amounts of thehalogen compounds are used to accelerate the reaction it is preferableto wash the calcined pigment to sub stantially eliminate suchwater-soluble compounds. If smaller amounts of the halogen compounds areused they are mostly fixed in the calcined lead silicate. Regardless ofthe amount of halogen compound used some of both basic element andhalogen remains in the finished pigment even though the pigment hasbeen-thor oughly washed with water. In view of the'comtile at thetemperatures used to form the lead silicate, do not greatly acceleratethe reaction. The chlorides, bromides and iodides of calcium,

barium, magnesium and zinc are very effective 'both as accelerators andstabilizers but of the three halogens, chlorine having a lower combiningweight permits the minimum addition of compound for any desired amountof the basic 'element.- The chlorides are also moreeconomicallyavailable.

Preferably I use the accelerating-stabilizing halogen compounds in theproportion of 0.2% to 1% of the lead silicate pigment calculated asbivalent oxide of the basic element, though even smaller amounts willeffect stabilization. Such 7 preferred amounts of stabilizing compoundsdefi- 'nitely accelerate the reaction; facilitate the use of lowreaction temperatures; only slightly change the chemical composition ofthe lead silicate and yield a stable pigment having a soft.

texture and good hiding power. Larger amounts of theaccelerating-stabilizing compounds, up to 5%, calculated as oxide of thebasic element, permit the reaction to be completed in 10 to 15 minutesif desired. The stabilizers should, preferably, not be used in excess of5% for, while larger amounts are effective, they unnecessarily changethe chemical composition of the pigment, reducing the percentage of leadsilicate, the characteristic properties of which it is desired toretain.

My method of stabilizing lead silicate pigments can be applied tosubstantially pure lead silicates of any desired composition. I havediscovered that, not Only can I make stable lead silicate pigmentsapproximating the regular molecular proportions such as acid leadsilicate, PbSlzOs, normal lead silicate, PbSiOs, and basic leadsilicate, P1325104, but I can also make equally useful lead silicatewith any basicity desired between the regular normal and basic formulaeor of any acidity desired between the regular acid and normal formulae.I can also make useful lead silicate pigments containing a higherpercentage of silica than the regular acid formula, PbSi205, but, thoughsuch a pigment is cheaper, its hiding power is reduced and thus there isa practical limit based upon utility of the product in its ap--plication. Such a practical limit appears to be an acid lead silicatecontaining not less than 85% actual lead silicate calculated as regularacid lead silicate, PbSizOs, and about 15% silica plus stabilizer. Theexcess silica may be chemically combined or in solid solution or,possibly, "partially in the free state.

Before giving detail examples showing the operation of my process andcharacteristics of the various products I shall give some general infor---mation relative to the raw materials, process and product,

The material used should be as free from impurities as possibleconsistent with industrial availability, since small amounts of iron orother impurity tend to discolor the product. Either -litharge (PbO) orother oxides of lead such as -red'lead (Pb-304) or lead peroxide (PbOz)may be used as a source of lead. Other lead compounds which yield theoxide upon heating may also be employed, such as white lead The silica.and stabilizing compounds supplying oxides of magnesium, calcium,barium, zinc, etc. should be as .pure as commercially obtainable andpreferably should .be finely divided to facilitate thoroughmixing andblending of the charge.

I first thoroughlymix the materials in the required proportions therebyreducing particle size and .also assuring a very close association ofthe ingredients. The milling may be done either in a batch or acontinuous type of mill; in either case by using a.porcelain or silexlining to avoid contamination of materials. If desired, the leadcompound and silica can be wet-milled and the slurry. from the milldewatered .as by settling and decantation and theaccelerating-stabilizing halogen compound then added and easily mixedwith the charge. water-soluble halogen compounds thus incorporated,while very effective stabilizers, are less effective .as accelerators ofthe reaction than is the case when incorporated by dry-mixing. Iattribute this difierence in effectiveness of the accelerator to thechemical reactions between the halogen compounds in solution and thelead compound; which reactions apparently must be reversed orsubstituted during the heating process before the reaction to form leadsilicate can be .completed. Therefore I prefer to mix the materialsincluding the halogen compound by drymilling. I

Either the wet pulp or a dry mixture is then chargedinto the heatingfurnace. The furnace may be either of the batch type such as a muffle,hearth furnace or Bruckner furnace, or of the continuous kiln type. Ineither case a lining of high-grade fire brick is satisfactory to avoidcontamination of materials. Whatever the type of furnace used, meansshould be provided for control of temperature. In furnaces whereproducts of combustion contact the charge a neutral or oxidizing flameshould be maintained to avoid possible reduction of metallic oxides.

Lead oxide and silica react very slowly .at a

temperature of 500 C. and at 550 C. a white perature range 650 C. forthe acid, 680 C. for

the normal and 725 C. for the basic lead silicate are exceeded for aconsiderable length of time. Thus, while my stabilized lead silicatepigments as a class can be made by the use of accelerators ,inatemperature rangexfromabout 500 C. to

about 700? .C;,:I prefer'to make all of them in a But I have found thatthe longer than appears absolutely necessary to incelerating andstabilizing compounds when apl and properties of the resulting whitelead silicate ing effect of halogen compounds by comparison temperaturerange of 500' to 600 0., since at such 1 effects, but the examples havebeen numbered temperatures the reaction is fairly rapid and'consecutively to facilitate positive reference. yields a product havinga soft texture and im- Tables I and II show examples of stabilizingproved hiding power. approximately normal lead silicates with com- Thetime required in the calcining zone of the pounds of each ofthe'halogens and also with the 5 furnace depends upon the temperatureand principal white-oxide-forming elements of group amount ofaccelerator used: the higher the tem- II of the periodic system incombination with perature within the permissible range the shorterhalogens; the stabilizing compound being used the time, but there is atendency toclearer color in larger amount to show the acceleratingefiect tones at the lower and medium temperatures notwithstanding theuse of some temperatures within that range. The time required tocomlower than practicable without the accelerating plete the reactionmay vary from 10 minutes to compound. several hours but it is usuallydesirable to hold Tables III and IV show the effectiveness of the chargeat the proper temperature a little medium and smaller amounts ofreaction-acsure that the reaction is complete. The progress plied toapproximately normal lead silicates.

of the reaction during calcination may be judged Table V shows theapplication of reaction-acby withdrawing a sample from the furnace forincelerating and stabilizing compounds in the prepspection. The finishedproduct should be subaration of basic lead silicates.

stantially white and should not acquire a yellow- Table VI shows theapplication of reaction-ac- 2o er or darker color upon milling orgrinding the celerating and stabilizing compounds in the prepsample in amortar. aration of acid lead silicates. Example 23 shows Lead silicatepigments, if not calcined at too the application of the method to a moreacid high a temperature, are soft in texture and fine lead silicate thanPbSizOa as well as the use of enough to be used as pigments for paint,but a lead compound instead of lead oxide in pre- :5

usually milling i required to break down aggreparing the pigment.Example 24 shows the apgates, reduce particle size and develop fulltinting plication of the method to a lead silicate interstrength andhiding power of the products. mediate in composition between the regularacid Either dry or wet-milling methods may be and normal products,PbSisOs and PbSiOs.

used with or without air separation or wet classi- Table VII, Examples25 and 26 show, by com- 30 fication respectively, thereby assuringuniformity parison, the advantage of using a reaction-ace of product. Ifthe calcined product contains celerating and stabilizing compound in theprepmore than 0.1 to 0.2% of water-soluble comaration of lead silicatepigments. From Example pounds it is preferable to wet-mill and wash the25 will be noted the comparatively short period pigment. The slurry isthen dewatered, dried, of time required to complete the reaction at the2:5

and if necessary passed through a disintegrator exceptionally lowtemperature shown and the to break down lumps and aggregates, hightinting-strength of the resulting product.

Examples showing the working of my process The examples in the tablesshow the acceleratpigments are Shown in Tables I to VII but such of thetime required to complete the reaction in examples should not beconstrued as limiting the 2 g g; f Eizamples application of my processsince many other lead i'. such 2 3 -1 Z1 i tfl h i t silicatepigments'can be made within the general maximuin acceleralgglg g s g ggi f of ,ii s P f i f g f d th reaction is more apparent by the rapiddevelope we 8 m lma ey en e e ment of a white color during thecalcination. oportlPns m the tables by dry'mlumg Thus it was noted thatin Examples 2, 3,, 6, 7, 8, except in Examples 1, 20 and 21, which werHand 18 the reaction was apparently complete blended y we -mi l afterheating from 10 to 15 minutes, though the The Tables I to VII have been,arranged for heating was actually continued for the longer convenience tgroup h examples. pproxiperiods shown in the tables to make sure themately, to show classes of products, reagents and reaction was complete.

Table I Example N0.

1 2 3 4 Normal lead silicate, PbSiOz Accelerating and stabilizing com-None Calcium chloride, CaCl Calcium bromide, CaBn= Calcium iodide, Calz=pound and amount. r 3% CaO. 3% CaO. 3% CaO. Mixture:

Lead oxide 78.80 76.50.. 76.60 76.50. Silica 21.20 20.60-. 20.50. 20.50.Stabilizer 5.95 11.01. 15.75. calcining condition Tune ...hours 2 0.50.5 l. Temp 0.- 600 to 6 l0 i l' 525 to 550 560 to 580 550 to 570.Treatment of calcined product Wet-m1lled 1% wet milled washed andWet-milled washed and Wet-milled washed and hours. drled. dried. dried.Ohem1csl composition:

L ead oxide percent 73.80 74.96. s01 "00.... 19.81 20.20. StablhZel doCaO 2.18 2.04. B1- 1 21 2.80 Spec fic gravity Tinting strength, whitelead stand- 105 ard=100. Color stab1lity under exposure toGrayinlflhours. Stable... Stable light. Dry pigment and pigment 7 groundin oil exposed to sunlight.

' Table i1 Example No.

Accelerating and stabilizing compound and amount. Mixture:

not.

Chemical composition:

Lead oxide percen Silica Stabilizer.

Specific gravity Tinting strength, whit cad standard=i0(l.

Color stability under exposure to light. Dry pigment and pigment groundin oil exposed to sunlight;

Barium chloride, BaOh-ZHzO BaO Wet-milled, washed and dried.

0.5 O 5 570 to 590 Magnesium chloride, MgCl:

=3% MgO Zinc chloride, ZHC12=3% ZnO.

525 to 550. Wet-milled, washed and dried.

Table III Example No.

Accelerating and stabilizing compound and 1 amount.

Mixture:

Lead oxide Stabilizer Calcining conditions:

Time "110218" enip Treatment of calcined product. a Chemicalcomposition:

Lead oxide percent 0....

Silica d Stabilizen. .d Specific gravity Tinting strength, white leadstandard=l00. Color stability under exposure to light. Dry pigment andpigment ground in oil exposed to sunlight.

Calcium CaO chloride, CaOl Magnesium chloride, MgClg:

1% MgO.

Zinc chloride, ZnCli= 1% ZnO.

Stable.

Calcium chloride, CaCli= 1. 570 to 580. Dry-milled.

Table IV Example No.

Accelerating and stabilizing compound and amount.

Mixture:

Lead oxide Sili Calcining conditions: Time. .hours. Teni Treatment ofcalcined product.

Chemical composition:

Lead oxide Tinting strength,

white lead standard=100.

Colorstabilityunder exposure to light. Dry pigment and pigment ground inoil exposed to sunlight.

Magnesium chloride, MgC12= 0.5% MgO.

Zinc chloride, ZnClz= 0.5% ZnO.

Calcium chloride, cacii= 0.2 CaO. =o.2 B o Dry-milled Barium Stable.

chloride, BaC1 .2H 0

Dry-milled.

B110, .14 01, present.

Presence of chlorine determined by qualitative test.

til

Tintin strength, white lead Table V Example N o.

Basic lead silicate,

PbiSiO,

Accelerating and stabilizing compound and amount. Mixture:

ca Stabilizer Calcining conditions:

Tune

Temp. Treatment uct O calcined prod- Chemical composition:

Lead oxide Oalcium chloride, OaCl:

=3% CaO Magnesium chloride MgOh 2 =37, MgO a inc None.

chloride, ZnOh 0.5% Zn Wet-milled 1 hours, dewatered and dried.

Specific gravi Tinting strengt standard=100 Color stability underexposure to light. Dry pigmer t and pigment ground in 011 exposed tosunlight.

Table VI 100. Darkened in 2 hours,

gray in hours.

Example N o.

Acid lead silicate. P bS iz0 Accelerating and stabilizing compound andamount. ixture:

Lead oxide Silica Calcium chloride, CaCh= 0.2% CaO.

Stabilizen- Calcium chloride, CaOh== 0.2% Geo.

W. L. sa.1o*= 55.00Pb0 4: 00

Calcium chloride, CaCli= Oalcining conditions:

Time ho nrsn Temp Treattment of calcined produc Chemical composition:

Lead oxide..-. percent Wet-milled and dried.

Wet-milled for 1 hour,

washed and dried.

Silica Stabilizer do Specific gravity stan ard= Color stabili underexposure to lig t. Dry pigment and pigment ground in oil exposed tosunlight.

Darkened after 10 hours exposure.

Wet-milled for I washed and dried.

l 570 to 590.

hour Dry-milled.

so ..II

Stable Stable Stable.

' White lead, 2PbQO:.PbHaOr, used to supply lead oxide. Presence ofchlorine determined by qual tetive test.

Table VI! Example No.

Accelerating and stabilizing compound and amount.

Mixture:

Oalcining conditions:

Time Temp Treatment of calcined product Chemical composition: Leadoxide... Silica Stabilizer. Specific gravity ard=100.

Color stability under exposure to light. Dry pigment and pigment groundin oil exposed to sunlight.

5 500 to 520 Dry-milled rium chloride Dry-milled.

Calcium chloride, CaCh None. 1% 0210.

680 to 700. T Dry milled.

Darkened in hours, gray in 12 hours.

9-, Gray-brown in 24hours.-

stable--.

Two classes of white pigments are generally recognized: (1) The opaquewhite pigments, such as white lead, zinc oxide and titanium pigmentswhich impart opacity and whiteness when mixed with oil or other organicvehicle: and (2) The extender pigments, such as whiting and china claywhich do not impart appreciable opacity when similarly mixed withvehicles.

The opaque pigments of class (1) are further sub-divided into twoclasses: (a) Thosewhich are reactive or capable of combining chemicallyor physico-chemically with ingredients of the vehicle, such as whitelead and zinc oxide: and (1)) those which are chemically inert, such astitanium pigments.

It is well known, that the reactive pigments have a special value, notpossessed by inert pigments, of enhancing certain desirable propertiesin paints and other products to which they are applied. For example,white lead has the property of combining with linseed oil and othervehicles to form a tough elastic coating which is very resistant tolight and weather when spread and exposed. Zinc oxide reacts with oiland other vehicles of paint products to harden the film or coatingthereby rendering it more resistant to abrasion as and when exposed ismore resistant to dirt collection.

My new lead silicate pigments belong to the class of opaque reactivepigments, though they differ in degree of reactivity as indicated by therate of drying when the pigments are mixed with raw linseed oil andexposed as a paint film.

The properties of my lead silicate pigments adapt them for applicationto ,oil, enamel and lacquer type paintsand for other purposes wherewhite pigments are used, such as for rubber, linoleum and other floorcoverings, printing inks, vitrified enamel, et cetera but I have foundthem particularly desirable for the preparation of exterior paints.

In the field of paints for exterior use much investigation has beenconducted for many years in an effort to combine the best features ofthe various opaque white pigments, but, while much progress has beenmade, the improvement in one feature has too often been accompanied by aloss of other desirable properties. Heretofore white lead carbonateappears to have been the only white opaque pigment which, withoutadmixture of other pigments, will make a reasonably satisfactoryexterior paint. Zinc oxide desirably hardens the paint film, but whenused without.

admixture of other pigments will check and crack: admixture with whitelead is the usual practice. Lithopone pigment because chemicallyunstable to atmospheric influences fails rapidly and is. little used inhigh-grade exterior paints.

Titanium pigments are in great demand for their excellent hiding power,but fail rapidly by chalking when used alone in exterior paints. Themanufacturers of paints now recommend admixture of titanium pigmentswith white lead, zincv oxide or both, thus improving the paint to acertain extent. However, titanium pigments with white lead and linseedoil vehicle while durable, rapidly accumulate dirt and mildew. Theaddition of zinc oxide prevents dirt collection and mildew, but in ashort time chalking takes place with consequent fading of tinted paints.Chalking and fading of tinted paints also occurs within a short timewhen mixtures of titanium pigments and zinc oxide are used with alinseed oil vehicle,

well as improving gloss,

From what I have heretofore stated about white lead silicate pigments,it is evident that hiding power, being in most cases in the order ofwhite lead in that respect, is not of special importance, though thehiding power is sufficient to make paints which will cover or hide thesurface with the usual number of coats. The outstanding characteristicof lead silicate pigments is their capacity to impart durability topaints.

I have discovered that paints made with my lead silicate pigments groundwith raw linseed oil vehicle are exceptionally durable as regards glossretention and resistance to checking and chalking of the paint film andare remarkably free from dirt collection and mildew. I have'alsodiscovered that these characteristics are imparted to paints containingmixtures of pigments one of which is a lead silicate. Thus combinationsof lead silicate with titanium pigments largely prevent the chalking,fading of tints, loss of gloss and dirt collection to which titaniumpaints are subject and prevent checking and cracking when combined witheither zinc oxide or lithopone pigments. The chalking which takes placewith white lead paints is also retarded.

The cleanness of surface and freedom from dirt collection and mildew ofpaints containing lead silicate I attribute to the reactive propertiesof the pigment resulting in paint films which dry to a relatively hardersurface.

For exterior paints made with mixed pigments and raw linseed oilvehicle, I prefer to have the lead silicate about 50% of the pigmentcombination, but lesser amounts may be used with good results andimprovement roughly proportional to the percentage used. I have alsofound the improvement of paints when lead silicate is added to aplurality of pigments, either a mixture of opaque pigments or a mixturecontaining a colored or tinting pigment or an extender pigment which isoften. added to increase the pigment volume concentration or to cheapenthe paint.

I have furthermore discovered that lead-silicates, especially basic leadsilicates, give remarkable protection to iron and steel under exposureto light, weather and other influence. This result I attribute to thereactive nature of the pigments, the lead silicates functioning likefree oxides of lead such as red lead which for many years has been usedwith linseed oil as paint where maximum protection of iron or steel isdesired.

Iclaim as my invention:

1. The method of making a substantially pure, white, anhydrous,light-stable lead silicate pigment, which consists in intimately mixinglead oxide, silica and a halide of an element of group II of theperiodic system the oxide of which is white, and then heating themixture at a temperature suiiiciently high to chemically combine theingredients in the solid phase without fusion to form said stabilizedlead silicate pigment.

2. The method of making a substantiallypure, white, anhydrous,light-stable lead silicate pigment, which consists in intimately mixinga lead compound which upon heating yields lead oxide, silica and ahalide of an element of group II of the periodic system the oxide ofwhich is white, and then heating the mixture at a temperaturesufficiently high to chemically combine the ingredients in the solidphase without fusion to form said stabilized lead silicate pigment.

3. The method of making a substantially pure,

. valent oxide of the element,

white, anhydrous, light -stable' lead silicate'pigment which consistsoxide, silica and a halide of one of the elements, magnesium, ca1ciu'm,barium,strontium and zinc in amount equivalent to not over 3% of thebivalent oxide of the element, and' then heating the mixture in thetemperature range 500 to 725 C. until the ingredients chemically combinein the solid phase without fusion to form said stabilized lead silicatepigment.

4. The method of making a substantially pure,

white, anhydrous, light-stablefilead silicate pigment which consists inintimately mixing lead oxide, silica and magnesium chloride in amountequivalent to not over 3% MgO and then heating the mixture in thetemperature range 500 to 725 C. until the ingredients chemically combinein the solid phase without fusion to form said stabilized lead silicatepigment.

5. The method of making a substantially pure, white, anhydrous,light-stable lead silicate pigment which consists in intimately mixinglead oxide, silica and calcium chloride in amount equivalent to not over3% CaO and then heating the mixture in the temperature range 500 to 725C. until the ingredients chemically combine in the solid phase withoutfusion to form said stabilized lead silicate pigment.

6. The method of making a substantially pure, white, anhydrous,light-stable lead silicate pigment which consists in intimately mixinglead oxide, silica and barium chloride in amount equivalent to not over3% BaO and then heating the mixture in the temperature range 500 to 725C. until the ingredients chemically combine in the solid phase withoutfusion to form said stabilized lead silicate pigment.

7. In the preparation of a substantially pure, white, anhydrous, leadsilicate pigment, the method of accelerating the reaction between leadoxide and silica in the solid phase, which consists in intimately mixinglead oxide, silica and a water-soluble halide of an element of group IIof the periodic system the oxide of which is white, and then heating themixture at a temperature sufficiently high to form the lead silicatepigment without fusion.

8. In the preparation of a substantially pure, white, anhydrous leadsilicate pigment, the method of accelerating the reaction between leadoxide and silica in the solid phase, which consists in intimately mixinga lead compound which upon heating yields lead oxide, silica and awater-soluble halide of one of the elements, magnesium, calcium, barium,strontium and zinc in amount equivalent to not more than 3% of biandthen heating the mixture in the temperature range 500 to 725C. to formthe lead silicate pigment.

9. In' the preparation of a substantially pure, white, anhydrous leadsilicate pigment, the method of accelerating the reaction between leadoxide and silica in the solid phase, which consists in intimately mixinglead oxide, silica and magnesium chloride in amount equivalent to notover 3% MgO, and then heating the mixture in the temperature range 500to 725 C. to form the lead silicate.

10. In the preparation of a substantially pure, white, anhydrous leadsilicate pigment, the method of accelerating the reaction. between leadoxide and silica in the solid phase, which consists in intimatelymixinglead oxide, silica and calcium chloride in amount equivalent tonot in intimately mixing lead method ofaccelerating the reaction betweenlead oxide and silica inthe-solid' phase, which consists in intimatelymixing lead oxide, silica and barium chloride in amount equivalent tonot over 3% BaO, and then heating the mixture in the temperature range500 to 725C. to form the lead silicate.

12. The method of making a substantially pure, white, anhydrous,light-stable lead silicate pigment, which consists in intimately mixinga lead compound which upon heating will supply to the product only leadoxide, silica and a halide of an element of group II of the periodicsystem the oxide of which is white, and then heating the mixture at atemperature sufficientlyhigh to chemically combine the ingredients inthe solid phase without fusion to form said stabilized lead silicatepigment.

13. In the preparation of a substantially pure, white, anhydrous, leadsilicate pigment, the method of accelerating the reaction between leadoxide and silica in the solid phase, which consists in intimately mixinga lead compound which upon heating will supply to the product only leadoxide, silica and a water-soluble halide of an element of group II ofthe periodic system the oxide of which is white, and then heating themixture at a temperature sufficiently high to form the lead silicatepigment without fusion.

14. The method of making a substantially pure, white, anhydrous,light-stable lead silicate pigment, which consists in intimately mixinga lead compound which upon heating will supply to the product only leadoxide, silica and a halide of an element of group II of the periodicsystem the oxide of which is white, heating the mixture at a temperaturesufficiently high to chemically com blue the ingredients in the solidphase without fusion to form said stabilized lead silicate pigment andfinally washing the calcined product to remove excess water-solublecompounds.

which is white, in amount equivalent to not over 3% of the bi-valentoxide of the element and also containing a chemically combined halogenin amount not greater than the chemical equivalent of the said element.

16. A substantially pure, anhydrous, lightstable, opaque, white pigmentconsisting essentially of lead silicate and containing chemicallycombined magnesium in amount equivalent to not over 3% MgO andchemically combined chlorine in amount not greater than the chemicalequivalent of the magnesium. 7

1 7. A substantially pure, anhydrous, lightstable, opaque, white pigmentconsisting essentially of lead silicate and containing chemicallycombined calcium in amount equivalent to not over 3% CaO and chemicallycombined chlorine in amount not greater than the chemical equivalent ofthe calcium.

18. A substantially pure, anhydrous, lightstable, opaque, white pigmentconsisting essentially of lead silicate and containing chemicallycombinedbarium in amount equivalent to not over.3% BaO and range 500 to725 Ci to form the.

substantially pure,

chemically combined chlorine in. amount not greaterthanl the chemicalequivalent otthebarium.

19. A substantially pure, anhydrous, lightstable, opaque, White pigmentconsisting essentially of lead silicate and containing 55- to 87% leadoxide, a chemically combined element from the group magnesium, calcium,barium, strontium and zinc in amount equivalent: to not over 3% of thebi-valent oxide of the element, a chemically combined halogen in amountnot greater than the chemical equivalent of the said element and thebalance silica.

LOUIS E. BARTON.

